Land-based mesoscale tempest
A 'land-based mesoscale tempest '''is a land-based convective system defined by rapid formation, a central dense overcast-type structure at the centre with significantly more intense convection and rainfall than the outer portions, and intensity ranging from near-unnoticeable light rain with virtually no wind to Category 5 hurricane intensity, and forward speeds ranging from virtually zero to more than 50 MPH. Formation On hypothetical planets with different atmospheric systems to Earth, LBMTs form during the formation stage of a cumulonimbus cloud or formation stage MCSes (excluding tropical cyclones and polar lows). During the formation stage, wind shear forcing on a cumulonimbus cloud or formation stage thunderstorm can cause the system's convection to be pushed into a circular shape, which then causes the thunderstorm's convection to mostly dissipate. After this, given enough moisture in the air, a very weak mesocyclone may form. This causes the cloud top temperatures and atmospheric pressures to slowly drop in the system as the mesocyclone starts to rotate faster. The most intense convection forms under the mesocyclone, creating the centre, while shallower convection surrounds the MCS and potentially in the spiral bands of the proto-LBMT, which are formed by the mesocyclone not having a perfectly circular shape, and often form if the system strengthens, as the mesocyclone often grows. Once the system is strong enough to meet the classifications outlined below it is declared an LBMT. Properties and Structure The average LBMT has a wind speed of 17 mph (27 kph), an average rain rate of 0.22 in/hr (5.58 mph) at the centre, a forward speed of 30 mph, and a lifespan of approximately ten hours. LBMTs have a low atmospheric pressure (with the pressure gradient tightening as the centre is reached), rainfall, and cold cloud tops. Stronger systems can have lightning, hail, and rarely tornadoes. Some of the strongest systems ever recorded have extremely small eye-like features, sometimes as small as 30 metres, although wind speeds of Category 5 hurricane caliber are often only reached for 10 to 30 minutes at a time. The average weak LBMT is 1 to 3 miles (1.6 to 4.8 km) wide, although systems as large as 100 mi (160 km) and as small as 300 metres (984 ft; 0.3 km; 0.18 mi) have been recorded. LBMTs grow as they strengthen, although systems wider than 15 miles (24.1 km) are rare, A typical LBMT consists of an exterior region of shallow convection, potentially with spiral bands harboring somewhat deeper convection, with a centre region of moderate or deeper convection located in the centre, spinning at anywhere from 6-8 MPH to over 250 MPH. A system is only considered an LBMT if: - The system has developed from either a cumulonimbus cloud system or a MCS, usually formation stage mutlicell or supercell thunderstorms and/or squall lines - The convective mesocyclone core is spinning faster than 5 mph (8 km/h) - The lowest cloud top temperature is lower than 0 °C (32 °F) - At least 1 mm per hour (0.04 inches per hour) of rain is falling in the centre, per radar or ground observer measurement. While LBMTs have some characteristics resembling hurricanes, there are several important differences. LBMTs almost entirely depend on shear and humidity, whereas shear weakens a hurricane. In addition, the systems are generally land based and get their energy from the mesocyclone, whereas hurricanes get their principal energy source from sea surface temperatures LBMTs can last for a very long time if sufficient moisture is present to power the convection. Some systems have even persisted over water for some time, although they almost exclusively form over land. Persistence times have been recorded from as low as two hours to several weeks, the average time being around six hours in non-tropical clim ates and two days in subtropical and tropical climates, with systems reaching wind speeds above 25 mph (40 km/h) having a far longer average lifespan of three days in non-tropical climates and five days in subtropical or tropical climates. LBMTs can strengthen and weaken extremely quickly if the conditions surrounding it change. Forecasting LBMTs can be difficult to forecast, so a nowcasting approach is typically used, with short-term 6, 12, 18, and 24 hour forecasts. Recently however, improved forecasting models have allowed relatively accurate forecasting of LBMTs out to 72 hours. Despite this, failures of LBMT forecasting can be very costly. One instance of this occurred on February 22, 2002. An LBMT with winds of 35 mph was expected to weaken and turn south, missing Birmingham, Alabama. In reality, due to problems forecasting the system and unexpected high humidity, the LBMT intensified, reaching an intensity of 65 mph, near-hurricane-force, before striking the centre of Birmingham at peak intensity, causing flooding and wind damage. The damage from the LBMT was estimated at $111 million. This incident caused the National Weather Service (at that time the National Hydrometerological Centre) to set up a division for forecasting LBMTs, becoming operational in 2005. Naming is scheduled to start in 2019 for severe LBMTs. The LBMT Scale In 2018, an experimental LBMT scale was created. It is scheduled to become operational in mid 2019. Effects Effects can range from none to extreme, depending on the LBMT's intensity. Intense systems can have winds in excess of 60 mph (100 km/h) or more, drop rain at violent rates, cause flash flooding, drop hen-egg-size (2 inches; 5.08 cm) or more hail, and in rare instances spawn weak (EF0-low-end EF1) tornadoes. They can be absorbed by landfalling hurricanes and tropical storms if they are near water, which under very rare conditions can spawn a supercell thunderstorm inside one of the spiral bands of the hurricane, potentially causing extremely strong tornadoes. Damages from LBMT's have ranged from $0 to multiple billion, the record being $3 billion. Radar values '''LBMT ': Wherever the system is an LBMT. Negative = is not, Positive = is. '''W: '''Local wind speed at the centre of the LBMT. EW: Environmental wind speed outside of the LBMT. Pressure: Local pressure in the LBMT. Generally relative to the environmental pressure, unless the system is strong, where it is absolute. Lightn/Lightning = Is lightning present If Lightning is present: Amount: The estimated amount of lightning, in strikes/minute. Hail: If hail is dropping and if so the observed size. Torn: If a tornado is present. WS: Wind strength. x (W - EW) is the first number, with y (the difference between W and W - EW) as the second, in the format x/y. ERR = Estimated rain rate in in/hr or mm/hr. Max DBZ = Maximum reflectivity of the system. Min CTT = Minimum CTTs of the system. Category:Scales